1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight unit for improving light efficiency and color realization ratio.
2. Discussion of the Related Art
In general, cathode ray tubes (CRTs) are commonly employed as display devices for televisions, machines, and information terminals. However, CRTs fail to meet the current trend of miniature and lightweight electronic products due to their size and weight.
Accordingly, many efforts have been made to study and develop various types of display devices as substitutes for CRTs, such as liquid crystal display devices (LCDs), plasma display panels (PDPs), electro-luminescence displays (ELDs), and vacuum fluorescent displays (VFDs). For example, the LCD devices have been actively developed as flat display panels in laptop computers, desktop computers, and large-sized information displays because of their high quality image, lightness, thinness, compact size, and low power consumption. Thus, the demand for the LCD devices increases continuously.
At this time, the LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. Further, the LCD panel includes first and second glass substrates bonded to each other at a predetermined distance, and a liquid crystal layer injected between the first and second glass substrates.
The first glass substrate (TFT-array substrate) includes a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction substantially perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions defined by the gate lines and the data lines in a matrix, and a plurality of thin film transistors (TFTs) switchable in response to signals on the gate lines for transmission of signals on the data line to the pixel electrodes.
The second glass substrate (color filter substrate) includes a black matrix layer for shielding portions of the first glass substrate excluding the pixel regions from extraneous light, a color filter layer (R/G/B) for displaying colors, and a common electrode for implementing a picture image. Then, a predetermined interval is maintained between the foregoing first and second glass substrates by spacers, and the first and second glass substrates are bonded by a sealant injected through a liquid crystal injection inlet.
FIG. 1 illustrates a related art backlight assembly. As shown in FIG. 1, the related art backlight assembly includes a fluorescent lamp 1, a light-guiding plate 2, a light-diffusion substance 3, a reflecting plate 4, a light-diffusion plate 5 and a prism sheet 6.
When a voltage is applied to the fluorescent lamp 1, electrons remaining in the fluorescent lamp 1 move to the anode, and the remaining electrons collide with argon Ar, whereby the argon Ar is excited. As a result, positive ions are generated, and the positive ions collide against the cathode, thereby generating secondary electrons. When the secondary electrons are discharged to the fluorescent lamp 1, the flow of the electrons collides with hydrargyrum vapor, and then ionized, thereby emitting ultraviolet rays and visible rays. Then, the emitted ultraviolet rays excite a fluorescent substance deposited inside the fluorescent lamp, thereby emitting the light.
The light-guiding plate 2 is a wave-guide that directs the light emitted from the fluorescent lamp 1 to be incident on the inside of the light guiding plate, and that emits light like a plate type light source. The light-guiding plate 2 may be formed of Polymethylmethacrylate (PMMA) having a great light transmittance. The light incidence of the light-guiding plate 2 is a function of the light-guiding plate thickness, the fluorescent lamp diameter, a distance between the light-guiding plate and the fluorescent lamp 1, and the shape of the reflecting plate. Generally, the fluorescent lamp 1 is slanted on the center of the light-guiding plate 2, thereby improving efficiency of the light incidence. The light-guiding plate 2 for the backlight unit of the LCD device may be divided into a printing-type light-guiding plate, a V-cut type light-guiding plate, or a scattering-type light-guiding plate.
Next, the light-diffusion substance 3 may be comprised of SiO2 particles, PMMA and solvent. At this time, SiO2 particles having porosity are used for diffusing the light. Also, PMMA is used for adhering SiO2 particles to a lower surface of the light-guiding plate 2. The light-diffusion substance 3 is deposited on the lower surface of the light-guiding plate 2 in a pattern of intervals, and the size of each deposit is gradually increased to obtain a uniform plate-type light source on an upper surface of the light-guiding plate 2. That is, the deposits have a small size near to the fluorescent lamp 1, and a large size away from the fluorescent lamp 1. At this time, the shape of the deposits may be varied. In case of the deposits having the same size, the respective deposits have a luminance of the same level regardless of the deposit shape.
Subsequently, the reflecting plate 4 is formed behind the light-guiding plate 2, whereby the light emitted from the fluorescent lamp 1 is incident on the inside of the light-guiding plate 2 which is on the side of the light guiding plate facing the reflecting plate 4. Also, the light-diffusion plate 5 is formed on the upper surface of the light-guiding plate 2, on which the dotted patterns are deposited, to obtain a uniform luminance at each viewing angle. The light-diffusion plate 5 may be formed of PET or Poly Carbonate (PC) resin, and a particle-coating layer is formed on the light-diffusion plate 5 for diffusing the light.
Next, the prism sheet 6 is formed to improve the frontal luminance uniformity of the light transmitted and reflected through the upper side of the light-diffusion plate 5. The prism sheet 6 transmits light of a predetermined angle, and the light incident at other angles is totally reflected, whereby the light is then reflected towards the lower side of the prism sheet 6 by the reflecting plate 4.
The backlight assembly having the aforementioned structure may be fixed to a mold frame, and a display unit disposed at an upper side of the backlight assembly is protected by a top case. Also, the backlight assembly and the display unit may be received between the top case and the mold frame and may be coupled to each other.
Hereinafter, a backlight unit of an LCD device according to the related art will be described with reference to the accompanying drawings. FIG. 2 illustrates a perspective view of a backlight unit using a related art fluorescent lamp.
As shown in FIG. 2, the backlight unit includes a fluorescent lamp 11, a lamp housing 12, a light-guiding plate 13, a reflecting plate 14, a light-diffusion plate 15, a prism sheet 16, a protection sheet 17, and a main supporter 18. A fluorescent substance is coated on the inside of the fluorescent lamp 11 for emitting light. Also, the lamp housing 12 fixes the fluorescent lamp 11, and concentrates the light emitted from the fluorescent lamp 11 in one direction. The light-guiding plate 13 directs the light emitted from the fluorescent lamp 11 to an upper side of an LCD panel, and the reflecting plate 14 adheres to a lower side of the light-guiding plate 13 and reflects any light directed away from the LCD panel back towards the light-guiding plate 13 and the LCD panel. The light-diffusion plate 15 is formed on an upper side of the light-guiding plate 13 to uniformly diffuse the light emitted from the light-guiding plate 13 towards the LCD panel. Also, the prism sheet 16 is formed on an upper side of the light-diffusion plate 15 to concentrate the light diffused in the light-diffusion plate 15, and to transmit the concentrated light to the LCD panel, and the protection sheet 17 is formed on an upper side of the prism sheet 16 to protect the prism sheet 16. The main supporter 18 receives and fixes the aforementioned elements.
In the aforementioned backlight unit, the light emitted from the fluorescent lamp 11 is concentrated on an incident surface of the light-guiding plate 13, and then the concentrated light passes through the light-guiding plate 13, the light-diffusion plate 15 and the prism sheet 16, whereby the light is transmitted to the LCD panel. However, the backlight unit using the related art fluorescent lamp has a low color realization ratio due to the light emitting characteristics of the light source. Also, it is hard to obtain a backlight unit having high luminance due to the constraints on size and capacity of the fluorescent lamp.
The backlight unit has been used for illuminating the screens of LCD devices, so that the viewer can watch information displayed on the screen in dark surroundings. Recently, many efforts have been made to obtain a very thin light-guiding plate to satisfy the demands of improved design and low power consumption. In addition, the LCD device has been developed to display various colors and to decrease the power consumption by using LEDs (light-emitting diode).
FIG. 3 illustrates a cross-sectional view of a backlight unit using a related art LED (light-emitting diode). As shown in FIG. 3, the backlight unit includes a light-guiding plate 21, an LED lamp 23, a lamp housing 22 and a reflecting plate 24. At this time, the light-guiding plate 21 is formed at the rear of an LCD panel (not shown), and the LED lamp 23 is formed at one side of the light-guiding plate 21 so as to emit the light. Also, the lamp housing 22 surrounding the LED lamp 23 reflects the light emitted from the LED lamp 23, and the reflecting plate 24 is provided below the light-guiding plate 21 so as to reflect the light leaking in a direction away from the LCD panel to the light-guiding plate 21.
The lamp housing 22 is formed of a reflective material such as aluminum (Al). Although not shown, the light emitted from the LED lamp 23 is reflected in the lamp housing 22, and then the reflected light is incident on the light-guiding plate 21. In this case, red, green, and blue LED lamps 23 are arranged in one-dimensional structures, and the LED lamps 23 are arranged on a PCB substrate 25 in the order of red, green and blue.
In case of displaying a picture image on the LCD panel using the aforementioned backlight unit, the LED lamps 23 are turned on. When a voltage is applied to the red, green and blue LED lamps 23, the three-colored LED lamps emit light. The light emitted from the red, green and blue LED lamps is scattered in the light-guiding plate 21, thereby generating a color mixture. As a result, the rear of the LCD panel is illuminated with a white light.
FIG. 4 illustrates a plan view of the backlight unit using the related art LED. As shown in FIG. 4, the backlight unit includes the LED lamps 23 and the light-guiding plate 21. The LED lamps 23 include red (R), green (G) and blue (B) LED lamps 23a, 23b and 23c, respectively, and the light-guiding plate 21 is formed at the rear of the LCD panel to uniformly diffuse the light emitted from the LED lamps 23 to the LCD panel.
In order to emit the white light using the LED lamps 23 as a light source, R, G and B monochromatic light is emitted from the respective LED lamps 23. In the area ‘a’ of the light-guiding plate 21, there is a predetermined portion 20 where the light emitted from the respective LED lamps 23 is not overlapped, so that it is impossible to emit the uniform white light. In the area ‘b’ of the light-guiding plate 21, R, G and B monochromatic light emitted from the respective LED lamps 23 is mixed, thereby generating uniform white light.
Accordingly, a luminous point is formed on the light-guiding plate 21 that effectively defines the ‘b’ area of the light-guiding plate 21 in the backlight unit where the light is effectively mixed, and thereby using the half of the light-guiding plate 21 set apart from the LED light source 23. By using LEDs as the light source for illuminating the LCD panel, it is possible to obtain miniaturization and low power consumption in electrical devices such as notebook PCs. A DC voltage of 1.5V is applied to the LED to produce light. Thus, DC-AC converter is not required, thereby greatly reducing power consumption. Also, the LED has greater reliability than that of a cathode ray tube because the LED is a semiconductor device. Furthermore, the LED can be miniaturized and have a long life.
In the backlight unit of the LCD device according to the related art, the light emitted from the red (R), green (G) and blue (B) LED lamps is mixed, thereby illuminating the LCD panel with the white light.
However, the backlight unit of the LCD device according to the related art has the following disadvantages.
It is difficult to mix the red, green and blue light emitted from the respective red (R), green (G) and blue (B) LED lamps, and to emit the white light by uniformly mixing the three colors, thereby reducing lighting efficiency and the color realization ratio.